Taps used for cutting or machining an internal thread in a workpiece are well known in the prior art. These taps which generally may be classified as a regular tap and a skip threading tap are distinguished one from the other by their thread pitch (S). To this end, the thread pitch of a skip threading tap is twice that of the thread pitch of a regular tap under circumstances that the pitch diameter pd and elevation angle (.beta.) of the two threads are the same. As the thread pitch of the internal thread of the workpiece to be machined is determined by the elevation angle of the thread of the tap, the thread pitch of the internal thread machined either by a skip threading tap or regular tap will be the same if the elevation angle of both taps are the same.
A skip threading tap may be preferred in use since the thread pitch of the skip threading tap, being twice that of the thread pitch of the regular tap, is capable of machining an internal thread in a workpiece with an efficiency two to three times that of the efficiency achieved using a regular tap, and with a service life of two to three times as well. This in all likelihood is because the occurrence of clogging with filings is reduced; and since the grip between workpiece and skip threading tap is reduced it is also possible to avoid any mashing and cracking of threading of a skip threading tap. The reduction in grip is a result of less friction.
According to the prior art three methods of forming a skip threading tap are known. A first of the methods is characterized as a grinding method done by hand during which every other or alternate thread of a regular tap is ground away. Another method concerns the automatic removal of threads by operation of a relieving machine. Similarly, every other or alternate thread of a regular tap is ground away. A third method concerns the direct formation of a skip threading tap by operation of a thread grinder. According to this method, a quenched tap block is placed directly on a thread grinder having auxiliary equipment to grind both thread and skip thread simultaneously. This last method may be preferred over the first two methods from the standpoint of better quality of product. The method, however, requires a much longer period of effort which oftentimes cannot meet development requirements of production.